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Spline couplings are commonly used to transfer rotary motion in rotating structures, such as low-pressure rotor systems in dual-rotor aero-engines. The misalignment introduced by spline coupling assembly error will seriously affect the safe operation of rotor system. In this paper, a method is proposed to calculate the spline meshing stiffness under arbitrary misalignment conditions, considering the static and dynamic misalignment. The accurate meshing parameters of spline couplings under parallel and angular misalignments were determined by computing the effective meshing region of misaligned spline teeth. Furthermore, a time-varying stiffness-damping spline model is established, considering friction on the spline teeth surfaces, to derive the dynamic model of the spline-rotor system under arbitrary misalignment conditions. The Newmark-β method, in conjunction with the Newton–Raphson method, was used to solve the dynamic response of the system, and the correctness of the model was verified by experiments. Finally, the variation of spline meshing stiffness and force caused by static and dynamic misalignment is described, and the effects of parallel and angular misalignment, as well as the phase difference in unbalance, on the vibration response and self-excited vibrations are also investigated. [ABSTRACT FROM AUTHOR]

Maneuverability is one of the important tactical and technical indexes of fighter aircraft. In this paper, the finite element method is used to establish a dynamic model of the rotor system that can consider arbitrary maneuvering flight form and rotor speed variation during the flight, and the vibration characteristic of the dynamic model is investigated in detail. In addition, the nonlinear forces caused by bearings and oil film are also considered. The Newmark-β method combined with Newton-Raphson method is adopted to solve the dynamic equations. The influences of speed variation, rolling, pitching, and yawing maneuver loads on the vibration responses of the rotor system are also evaluated. [ABSTRACT FROM AUTHOR]

The geometric nonlinearity due to static and thermal load can significantly alter the vibration response of structures. This study presents a semi-analytical approach to illustrate the nonlinear vibration of clamped-clamped beams under static and thermal loads. The von Karman strain and Hamilton's principle are employed to derive the nonlinear static equilibrium equation and nonlinear governing equation. The vibration equation's coefficient is variable. The transfer-matrix method and local homogenization are used to solve the equation. The proposed method's accuracy is validated by commercial software and literature. The numerical results indicate that uniform stress caused by thermal load only reduces the structural mode frequencies. The geometric nonlinearity of the structural static deformation affects both the mode frequencies and mode shapes. And the mode shapes cannot be approximated by harmonic functions. When the static deformation is significant, the structure's local RMS response is substantially affected. The combined loads have a more significant impact on the acceleration response than the superposition of individual load effects. [ABSTRACT FROM AUTHOR]

In this paper, a flexible tether-net system is applied to capture the space debris and a numerical framework is established to explore its nonlinear dynamic behaviors, which comprises four principal phases: folding, spreading, contacting, and closing. Based on the discretization of the whole structure into multiple nodes and connected edges, elastic force vectors and associated Jacobian matrix are derived analytically to solve a series of equations of motion. With a fully implicit method applied to analyze the nonlinear dynamics of a slender rod network, the involved mechanical responses are investigated numerically accounting for the interactions. Contact between the deformable net and a rigid body is handled implicitly through a cost-effective modified mass algorithm while the catenary theory is utilized to guide the folding process (from planar configuration to origami-like pattern). The dragging and spreading actions for the folded hexagon net could be realized by shooting six corner mass toward a specific direction; next, the six corners would be controlled to move along a prescribed path producing a closing gesture, when touch between the flying net and the target body is detected, so that for the space debris could be captured and removed successfully. We think the established discrete model could provide a novel insight in the design of active debris removal (ADR) techniques and promote further development of the model-based control of tether tugging systems. [ABSTRACT FROM AUTHOR]

Ground simulation of space quasi-zero gravity environment is essential in modal testing of space structures. The air flotation method is widely used, in which the air cushion unit is attached to the measured structure. This inevitably increases the total mass of the structure to be tested and leads to the incorrect prediction of modal parameters. And irregular airflow disturbances of the air cushion unit also have negative effects on the test. The fast elimination method is presented to remove the adverse impact in the test data based on measured frequency response functions. Numerical simulations are performed employing the magnetometer structure. Results show that this method effectively removes the adverse mass and the airflow effect. [ABSTRACT FROM AUTHOR]

Aiming to solve the complicated problem about the coupling effect of aerodynamic force and heat for hypersonic vehicle, one-way coupling was applied to decouple the environmental load. Then, the distribution of aerodynamic force and heat of a typical composite wing under the flight situation of Mach 5 to Mach 8 was studied by finite element analysis. Based on the classical flutter theory, the critical flutter boundary was obtained. The results show that flutter phenomenon is caused by first-order bending-torsional coupling. Considering the fabricability of the composite wing, the aerothermoelastic tailoring design of composite wing under typical flight situation was carried out. In the optimal process, static intensity and thermal flutter velocity are considered as the constraints, and the optimization target is to obtain minimum mass. For approaching the best structural dynamic characteristics and aeroelastic effect, the bending-torsional coupled stiffness of the composite structure is regulated through changing layers, paving angles and laying sequence. Based on C/C ++ open source environment, a software called Aero-Opt was developed to construct an independent-industrialized analysis platform, and the aerothermoelastic tailoring research of the composite wing under most severe aerodynamic heating condition was carried out. After optimization, the thermal flutter boundary of the composite wing is improved by 22.4%, while its mass is reduced by 15.76% . [ABSTRACT FROM AUTHOR]

This paper studies the yield behavior of a woven carbon-fiber-reinforced silicon-matrix (C/SiC) composite under dynamic tensile loading. Experiments were carried out to obtain the tensile properties of the C/SiC composite at a strain rate range of 2 × 10−5/s to 99.4/s. A strain-rate-dependent yield criterion based on the distortional strain energy density theory is established to describe the yield behavior. The interval uncertainty is considered for a more reliable yield prediction. Experimental results show that the yield stress, elastic modulus, and yield strain of the C/SiC composite grow with the increasing strain rate. The failure mode transitions from progressive crack extension to uneven fiber bundle breakage. The predicted results by the yield criterion match well with experimental data. Experimental results are enveloped within the uncertainty level of 45% in the critical distortional energy density, corresponding to an uncertainty of 14% and 11% in the yield stress and yield strain, respectively. With the support of the proposed strain-rate-dependent yield criterion, the yield behavior of the C/SiC composite under dynamic loading conditions can be predicted with reasonable accuracy. [ABSTRACT FROM AUTHOR]

A double-well Duffing oscillator with nonlinear damping is identified in this paper, using the nonlinear subspace identification (NSI) technique in conjunction with a novel virtual input perturbation approach. This kind of system exhibits a negative stiffness characteristic, which has become popular in engineering applications such as vibration absorbers. The studied device exhibits strong nonlinear elastic and damping behaviors. In particular, the latter results in a very challenging identification, and the applicability of the proposed method to nonlinear damping is therefore discussed. In this context, the method is first tested considering simulations with quadratic friction. The experimental investigation is eventually undertaken considering a broadband random excitation, with a friction model depending on both velocity and displacement. Results show that NSI is capable of estimating the nonlinear model associated with stiffness and damping nonlinearities with a high level of confidence. This is particularly true in the case of complex nonlinear behaviors related to the double-well characteristics, using the proposed virtual input perturbation technique. This is confirmed also by the proposed experimental application and enlarges the range of applications of the NSI technique. [ABSTRACT FROM AUTHOR]

Rolling bearing and squeeze film damper will introduce structural nonlinearity into the dynamic model of aeroengine. Rubbing will occur due to the clearance reduction design of the engine. The coupling of structural nonlinearity and fault nonlinearity will make the engine present rich vibration responses. This paper aims to analyze the nonlinear vibration behavior of the whole aeroengine including rolling bearing and squeeze film damper under rubbing fault. Firstly, the dynamic model of a turboshaft engine with nonlinear support and rubbing fault is established; The rolling bearing force, the oil film force and the rubbing force are introduced into a dual-rotor–casing model with six support points. Secondly, the linear part of the model is verified by the dynamic characteristics of the three-dimensional finite element model. Finally, the varying compliance vibration, the damping effect and the bifurcation mechanism are analyzed in detail in which the bearing clearance, speed ratio and rubbing stiffness are considered. Results show that the rubbing fault in the nonlinear support case will excite more significant varying compliance vibration in the low-speed region and expand the rotating speed range of the chaotic region in the high-speed region compared with that in the linear support case. [ABSTRACT FROM AUTHOR]

Based on the long short-term memory (LSTM) neural network, an identification method is proposed to identify the support stiffness of an aero-engine at rotating state. First, the dynamic model of a whole aero-engine with nonlinear support was established. The displacement responses corresponding to different support stiffnesses at the target rotating speed were obtained. Then, the deep leaning neural network with LSTM as the core layer was established. The network was trained with the displacement responses as the inputs and the support stiffnesses as the outputs. The nonlinea relationship between the support stiffnesses and the displacement responses was constructed. Finally, the support stiffness was directly identified with the generalization of the deep leaning network. The support stiffness of an aero-engine was identified by the proposed method. Results show that the recognition error is less than 2%, and the recognition accuracy of LSTM is better than that of the radial basis function neural network and support vector machine. The proposed method can avoid the complex optimization process in inverse dynamic problems and realize the identification of dynamic parameters for complex nonlinear structures. [ABSTRACT FROM AUTHOR]

Compared with the three-dimensional rotor model for a central tie rod rotor, an equivalent one-dimensional model can greatly improve the computational efficiency in rotor dynamics analysis with a certain accuracy. However, little research work can be found on improving the modeling accuracy of one-dimensional models using experimental data. In this paper, a one-dimensional discrete mass model considering pretightening force is proposed for central tie rod rotors to achieve the purpose of both efficient and accurate modeling. Experimental testing and three-dimensional model analysis are used as reference and verification approaches. A sensitivity-based method is adopted to update the proposed one-dimensional model via minimizing the error in the critical speed comparing with the corresponding three-dimensional finite element model which has been verified by a modal test. Prediction of damped unbalanced response is conducted to show the practicality of the updated one-dimensional model. Results show that the method presented in this research work can be used to simulate a complex preloaded rotor system with high efficiency and accuracy. [ABSTRACT FROM AUTHOR]

An improved finite element modeling method is developed for isotropic curvilinearly stiffened plates under a thermal environment. The existing modeling method for curvilinearly stiffened plates avoids the difficulty of node overlap, which is suitable for plates with different thicknesses. By introducing the influence of temperature on material parameters and thermal stress on additional stiffness, the existing method is improved and extended to the study of thermodynamics. The proposed method is verified by modal test at normal temperature and commercial finite element software at the thermal environment. The variation of dynamic characteristics with temperature under different boundary conditions was studied. Results show that when the boundary conditions are asymmetric, the influence of temperature on the thermal mode shape is more significant than the case of symmetrical. [ABSTRACT FROM AUTHOR]

The effect of geometrical nonlinearities due to pressure load on the thermal buckling and dynamic characteristics of composite plates are investigated in this paper, which is the main contribution of this research work. The mechanical behavior of the plate is described with the first-order shear deformation theory. The geometrical nonlinearity due to both thermal effect and pressure load is introduced in the finite element model of the plate via additional stiffness matrices. Thermal buckling and modal analysis of a four-sided simply supported rectangular composite plate under different pressure fields are conducted. Numerical results show that both the mode frequencies and critical buckling temperature of the plate rise with the increase of the pressure. The vibrational mode shapes change with the gradient pressure load field. The maximum buckled deflection point moves from the center to the place where is easier to reach compressive stress state under uniform thermal load. The pressure distribution has a significant effect on the buckling mode shapes of the plate. [ABSTRACT FROM AUTHOR]

In aerospace engineering, structural modifications play an essential role in design of structures. In some cases, it is necessary to guarantee that a specific natural frequency of the structure remains unchanged when additional masses are attached. The methods based on the Sherman-Morrison formula are proposed in this paper, called the optimal selection method and the absolute value method, to maintain the specific natural frequency. The methods are both implemented by installing a spring on the system and can eliminate the effect of the additional mass on the specific frequency. The proposed methods were verified to be effective and accurate through numerical simulations. Results show that the optimal selection method has similar applicability as the existing real value method, and both methods are applicable only in cases of small damping. In addition, the absolute value method has extensive applicability in systems with either small or large damping. [ABSTRACT FROM AUTHOR]

The effective numerical model of stitched sandwich composite plays crucial role in dynamics analysis and structural design. A sensitivity-based multistage model updating method is proposed for modeling of the stitched sandwich panel using the experimental modal frequencies. Based on applying the periodic boundary condition, the initial equivalent finite element model of a stitched sandwich composite is constructed; the measured frequencies are obtained from the modal test. According to relative sensitivity analysis of modal frequencies with respect to updating parameters, the different well-conditioned groups of parameters are selected to be updated. This method is applied to a stitched sandwich panel with established configuration of the stitches. Results show that the proposed method with the smallest condition number of relative sensitivity matrix has a better performance than the multistage method using type-based parameter group selection and the traditional model updating approach. [ABSTRACT FROM AUTHOR]

Modal parameter identification is adversely affected by the mass loading of the transducer in experiments, especially when multi-transducers are arranged on the lightweight structure. In order to remove the coupling effects of transducers on each measurement point, a hierarchical multi-transducers eliminations method based on Sherman–Morrison theory is investigated. The method consists of two steps: (1) Decomposition: multiple elimination is decomposed into multi-levels, the relationship of the frequency response functions between each level is illustrated in the tree diagram; (2) Elimination: according to the relationship between each level, the measured frequency response functions are modified level by level. Numerical simulation is conducted by employing a three-degrees-of-freedom spring-mass system and the robustness is verified in the noise case. Experimental investigations are undertaken by employing a lightweight cantilever beam: Laser Doppler vibrometer is adopted to obtain measured frequency response functions without transducer mass loading effect, which are regarded as the target data. The initial frequency response functions are obtained in the case, in which multi-accelerometers are arranged and the effects should be removed. The result shows that the method can effectively decouple the frequency response functions due to transducers. In the elimination process, it is necessary to delete duplicate information (frequency response functions), which can greatly reduce the amount of calculation. And the effects of multi-transducers mass can be removed and the corrected frequency response functions are in quite good agreement with the target values. [ABSTRACT FROM AUTHOR]

Modal parameter identification is adversely affected by the mass loading of the transducer in experiments, especially when multi-transducers are arranged on the lightweight structure. In order to remove the coupling effects of transducers on each measurement point, a hierarchical multi-transducers eliminations method based on Sherman–Morrison theory is investigated. The method consists of two steps: (1) Decomposition: multiple elimination is decomposed into multi-levels, the relationship of the frequency response functions between each level is illustrated in the tree diagram; (2) Elimination: according to the relationship between each level, the measured frequency response functions are modified level by level. Numerical simulation is conducted by employing a three-degrees-of-freedom spring-mass system and the robustness is verified in the noise case. Experimental investigations are undertaken by employing a lightweight cantilever beam: Laser Doppler vibrometer is adopted to obtain measured frequency response functions without transducer mass loading effect, which are regarded as the target data. The initial frequency response functions are obtained in the case, in which multi-accelerometers are arranged and the effects should be removed. The result shows that the method can effectively decouple the frequency response functions due to transducers. In the elimination process, it is necessary to delete duplicate information (frequency response functions), which can greatly reduce the amount of calculation. And the effects of multi-transducers mass can be removed and the corrected frequency response functions are in quite good agreement with the target values. [ABSTRACT FROM AUTHOR]

As research works of the transient statistical energy analysis (TSEA) and transient local energy approach (TLEA) mostly focus on simple structures, TSEA and TLEA are adopted to quantify the transient response of a complex vibro-acoustic system at the mid-high frequency range in this paper. Numerical examples of a coupled oscillator system, an L-shaped plate, and a launch vehicle fairing model are conducted to demonstrate the effectiveness and accuracy of TSEA and TLEA. The computational precision of TSEA and TLEA is verified by the analytical solution and finite element method. Furtherly, the transient energy responses of subsystems with different coupling ratios between subsystems are investigated. Results show that TLEA has a better performance than TSEA. With the increasing coupling ratio between subsystems, the rise time and peak energy of transient energy response of subsystems decrease gradually. Both ratios of rise time and peak energy predicted by TLEA to these of the TSEA increase as the rising of the coupling ratio. [ABSTRACT FROM AUTHOR]

An approach on determining elastic parameters and Coefficient of thermal expansion (CTE) of a 2.5-dimensional (2.5D) braided composites is proposed in this paper, by adopting mesoscopic mechanics integrated Finite element (FE) modeling. According to the geometric features of meso-structure, Representative volume cell (RVC) models of composite for predicting thermo-elastic parameters are established. On the basis of the models, homogenized parameter prediction is carried out in three steps: Firstly, equivalent elastic properties is predicted subject to periodic displacement boundary conditions; secondly, the equivalent thermal modulus is determined by using the periodic non-adiabatic temperature boundary conditions; thirdly, using the obtained elastic parameters and thermal modulus to calculate the equivalent coefficient of thermal expansion. A multiscale finite element analysis is conducted: The thermo-elastic parameters of yarn is calculated using the RVC model; subsequently, the equivalent parameters of the yarn are substituted into the RVC of 2.5D braided C/SiC composites, to predict the thermo-elastic parameters. Results indicate that the CTE determined by homogenized parameter prediction of 2.5D C/SiC composites shows good agreements with the experimental results. At last, the refined FE model and the equivalent homogeneous model are employed to verify the effectiveness of the predicted parameters in terms of effective modeling. After the comparative analysis on thermal modal data between refined model and equivalent model, the acquired results demonstrate the effectiveness of the proposed method in determining thermo-elastic parameters. [ABSTRACT FROM AUTHOR]

Random-vibration fatigue evaluation can be of considerable importance in the design phase of aerospace structures due to the severe dynamic loads in service. This paper presents the utilization of modal stress approach to the issue of structural random-vibration fatigue evaluation. Prognosis of random fatigue hotspots by using stress mode shapes is theoretically demonstrated. A two-step procedure is proposed for computational efficiency. Firstly, modal stress analysis is conducted to locate the fatigue hotspots in a dynamic structure. Secondly, the frequency domain-based approach for random fatigue evaluation is performed at these hotspots, as opposed to the computation of the entire structure as before. The capability of stress mode shapes to locate fatigue hotspots is verified by numerical investigations. The finite element model of a single-lap plate structure containing various opening holes was constructed for case study. Six elements were identified as hotspots by using modal stress distributions. Then, random responses and fatigue evaluation of the entire structure were carried out for verification. Good agreement was observed between the fatigue damage contour and the modal stress distributions, which can indicate that the critical positions predicted by stress mode shapes have good accuracy. The calculation time and storage space can be significantly reduced by means of the proposed evaluation procedure. Therefore, the accuracy and efficiency of utilization of modal stress approach in random fatigue evaluation can be ensured. [ABSTRACT FROM AUTHOR]

Substructure method has been widely applied in dynamic analysis of complex structures due to high computational efficiency. On the basis of Residual flexibility mixed-boundary (RFMB) substructure method, a model updating approach is proposed in this paper. Four major steps of the RFMB model updating method are summarized as: 1) Substructuring: Dividing the whole structure into residual part and reduced part according to the junction surface; 2) reduction: Using the RFMB component mode synthesis approach to reduce the order of each substructures; 3) assembly: Residual structure analysis by using the reduced assemble matrix; 4) updating: Model updating by solving the optimization problem. Numerical simulation is conducted to verify the effectiveness by adopting a cantilever plate in case I. In case II, the proposed method is applied to identify the elastic parameter of interface of a bolted joint structure using experimental data. After parameter identification, the maximum error between numerical results and the experimental data decreases to 2.44 %. And three component mode synthesis model updating methods: Craig-Bampton (CB), Mixed-boundary (MB) without considering residual flexibility and RFMB model updating approach, are applied to update the same bolted joint structure for comparing the accuracy. For comparing the computational efficiency, the RFMB model updating approach is applied to the complicated aero-engine casing structure. In case III, the average time-consuming of the Whole finite element model (WFEM) is 5.15 times to the Residual finite element model (RFEM) in the single updating iteration. Results show that the proposed approach has better performance in the finite element model updating. [ABSTRACT FROM AUTHOR]

Design optimization of dynamic properties, for example, modal frequencies, can be of much importance when structures are exposed to the shock and/or vibration environments. A modal strain based method is proposed for fast design of natural frequencies of plate-like structures. The basic theory of modal strains of thin plates is reviewed. The capability of determining the highly sensitive elements by means of modal strain analysis is theoretically demonstrated. Finite element models were constructed in numerical simulations. Firstly, the application of the proposed method is conducted on a central-massed flat plate which was topologically optimized by the Reference. The results of modal strain analysis at the first mode have good agreement with the results from the topology optimization. Furthermore, some features of the strain mode shapes (SMSs) of the flat plate are investigated. Finally, the SMSs are applied to the optimization of a stiffened plate. Attention is focused on the distributions of the SMSs of the stiffeners, which also shows good agreement with the results from the topology optimization in the previous study. Several higher orders of SMSs are extracted, which can visualize the most sensitive elements to the corresponding modal frequency. In summary, both the theory and simulations validate the correctness and convenience of applying SMSs to dynamic design of plate-like structures. [ABSTRACT FROM AUTHOR]

Based on the assumption of quasibrittle failure under high-cycle fatigue for the metal material, the damage constitutive equation and the modified damage evolution equation are obtained with continuum damage mechanics. Then, finite element method (FEM) is used to describe the failure process of metal material. The increment of specimen’s life and damage state can be researched using damage mechanics-FEM. Finally, the lifetime of the specimen is got at the given stress level. The damage mechanics-FEM is inserted into ABAQUS with subroutine USDFLD and the Python language is used to simulate the fatigue process of titanium alloy specimens. The simulation results have a good agreement with the testing results under constant amplitude loading, which proves the accuracy of the method. [ABSTRACT FROM AUTHOR]

Runyang Bridge is a newly built cable-supported bridge that crosses the Yangtze River in China. The bridge is composed of one suspension bridge and one cable-stayed bridge. During the construction of the bridge, a structural health monitoring system (SHMS) was installed, which was designed by the Southeast University. Since the bridge was open to traffic, quantities of structural ambient responses have been recorded by the SHMS. And, it’s really important to extract structrural information from these records for health monitoring. This paper presents the study on modal identification of the bridge. The dynamic properites including modal frequencies, mode shapes and damping ratios are extracted from the ambient responses. Two identification methods are employed including the enhanced frequency domain decomposition and the stochastic subspace identification. The identified modal parameters from the two methods are compared. Results show that modal frequencies and mode shapes from the two methods are almost the same while the damping ratios are different. [ABSTRACT FROM AUTHOR]